DURABILITY OF GLUED WOOD 

TO METAL JOINTS 

Information Reviewed and Reaffirmed 

October 1954 

This Report is One of a Series 

Issued in Cooperation with 

AIR FORCE-NAVY-CIVIL SUBCOMMITTEE 

on 

AlRCRAFT DESIGN CRlTERlA 

Under the Supervision of the 

AIRCRAFT COMMITTEE 

of the 

MUNITIONS BOARD 

No. 1570 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

Madison 5 , W i s c o n s i n 

In Cooperation with the University of Wisconsin 
DURABILITY OF GLUED WOOD TO METAL JOINT1 

By 

HERBERT W. EICKNER, Engineer 

This study on the durability of glued wood-to-metal joints subjected 

to various exposure conditions was conducted at the request of the ArmyNavy-Civil Committee on Aircraft Design Criteria. The purpose of the study 

was to gain information on the comparative durability properties of wood-tometal joints made with different commercial adhesives, so that they might be 

more widely and safely used in aircraft construction. Exposure conditions 

were selected to measure resistance of painted and unpainted aluminum-towood and steel-to-wood joints to water, high humidities, high and low 

temperatures, aircraft fluids, and weather. 

Type of Specimen 

The type of specimen used for this study consisted of a 1- by 2-inch 

piece of 0.037-inch (20 gauge) cold-rolled steel or 0.032-inch aluminum-clad 

aluminum alloy lap-jointed 1/2-inch with a 1-1/4- by 2-inch piece of threeply, 3/16-inch yellow birch plywood (fig. 1). With one of the adhesives 

(glue C of table 1), some specimens were prepared by direct bonding of the 

metal and. birch, and other specimens with a 1/28-inch shim of walnut veneer 

in the lap area between the metal and the birch plywood. In one-half of the 

specimens, the metal portion was protected with paint. 

Glues and Gluing Processes 

Six commercial gluing processes formuated for use in bonding wood to 

metal were included in this study. With two of these gluing processes, a 

woodworking resorcinol resin was used as a secondary glue. These gluing 

processes are designated by numbers and the glues by letters 

as given in table 1. 

1 This is one of a series of progress reports prepared by the Forest Products 

Laboratory relating to the use of wood in aircraft. Results here reported 

are preliminary and may be revised as additional data become available. 

Report No. 1570 -1-
The processes by which the glues were employed differed according 

to the manufacturer's instructions. Glues A, C, and F were used by themselves in direct bonding. Glues D and E were used together in direct 

bonding. Glue G was used as the secondary glue with glue B and, in process 

6, as a secondary glue for bonding the birch to the walnut shim. 

Test Procedures 

Preparation of Metal for Gluing 

Gluing Process 1.--One- by 2-inch pieces of steel and aluminum-clad 

aluminum alloy were placed in a jig and 9/64-inch locating holes drilled 

into them (fig. 2). Burrs formed in drilling were ground off. 

The steel pieces were degreased by immersion for 4 to 6 minutes in 

a hot (160° to 180° F.) trisodium phosphate solution prepared by saturating 

warm water with trisodium phosphate and adding 0.5 gram of wetting agent 

per 100 cubic centimeters of solution. The solution was rinsed from the 

pieces with cold tap water and then with hot distilled water. 

After degreasing, the steel pieces were etched for about 5 minutes 

in an acid solution of the following composition: 

Parts by volume 

Concentrated hydrochloric acid 50 

30 percent hydrogen peroxide 2 

Formalin 10 

Distilled water 45 

If 5 minutes' immersion was not sufficient time to remove all evidence of 

rust or mill scale, the etching treatment was continued as long as necessary. 

Immediately after etching, the metal pieces were rinsed in flowing tap 

water, immersed in hot distilled water, dipped in acetone, and air dried. 

The small aluminum pieces were wiped with a cloth wet with acetone to 

remove the identification lettering. They were then degreased by immersing 

in a hot (160° to 180° F.) bath of the following composition: 

Sodium metasilicate 3.6 ounces 

Wetting agent .4 ounce 

Distilled water 1.0 gallon 

The pieces were immersed until the film of wash tap water showed no "break" 

as it drained. The period of immersion ranged from 7 to 12 minutes. After 

degreasing, the pieces were rinsed successively in cold tap water, hot distilled water, and acetone, and finally air dried. 

Report No. 1570 -2-
Gluing process 2.--For gluing with this process, 8- by 8-inch metal 

squares ware degreased and etched preparatory to application of glue and 

cutting into 1- by 2-inch pieces. 

The steel squares were degreased and etched in the same manner as 

described for the small steel pieces under Gluing Process 1. 

The aluminum squares were wiped with an acetone-soaked cloth to 

remove the identification lettering, degreased in sodium metasilicate solution in the same manner as described under Gluing Process 1, then immersed 

for about 20 minutes at 140° to 150° F. in an acid bath of the following 

composition: 

Parts by weight 

Sodium dichromate 1 

Concentrated sulphuric acid 10 

Distilled water 30 

After etching, the pieces were rinsed in tap water, immersed in hot distilled water, dipped in acetone, then air dried. 

Gluing Process 3.--Steel sheets, 8 by 8 inches in size, were degreased 

and etched in accordance with the recommendations by the manufacturer of 

glue C (Process 3). The degreasing was done by immersion for 5 to 7 minutes 

in a boiling bath of the following composition: 

Sodium carbonate 2.0 ounces 

Sodium hydroxide 2.0 ounces 

Wetting agent 0.5 ounce 

Sodium metasilicate 3.0 ounces 

Laundry soap 0.6 ounce 

Distilled water 1.0 gallon 

After rinsing in cold tap water, the sheets were immersed for 2 to 4 minutes 

at room temperature in a pickling solution of the following composition: 

Parts by volume 

Concentrated sulphuric acid 10 

Concentrated nitric acid 10 

Distilled water 80 

The steel sheets were rinsed in tap water and then immersed for approximately 

1 minute at room temperature in a "bright" acid bath of the composition: 

Parts by volume 

Concentrated hydrochloric mid 60 

30 percent hydrogen peroxide 2 

Distilled water 38 

Report No. 1570 -3-
Afterwards, the sheets were rinsed in running tap water, immersed in hot 

water, dipped in acetone, and air dried. 

The aluminum sheets were degreased in a sodium metasilicate solution 

and etched in sodium dichromate-sulphuric acid in the same manner as 

described for Gluing Process 2. 

Gluing Process 4.--One by 2-inch pieces of metal were drilled and 

deburred as described under Process 1. The steel pieces were degreased and 

etched as in Gluing Process 1. The aluminum was degreased as in Gluing 

Process 2. 

Gluing Process 5.-Thedrilling and deburring of metal pieces was 

done as described under Gluing Process 1. The steel was degreased and etched 

as in Gluing Process 1. The aluminum pieces were cleaned as in Gluing 

Process 2. 

Gluing Process 6.-Themetal test pieces were prepared as in Gluing 

Process 3. 

Preparation of Plywood and Veneer 

Gluing Processes 1, 2, 3, 4, and 5.--Selected aircraft-grade 1/16-inch 

yellow birch veneer was bonded with phenol-resin film glue into three-ply, 

3/16-inch plywood. The plywood panels were conditioned for at least 1-week 

at 80° F. and 30 percent relative humidity, then cut into pieces 3 by 8 

inches in size, with the grain direction of the face plies parallel to the 

shorter dimension of the piece. The portion of plywood to which the metal 

was to be bonded was lightly sanded with No. 00 garnet sandpaper. 

Gluing Process 6.--In this process, 1/28-inch sliced walnut veneer was 

used as a shim between the metal and plywood. The walnut veneer was conditioned to equilibrium at 30 percent relative humidity and 80° F. , then cut 

into 1/2-inch strips about 9 inches long, with the grain direction in the 

shorter direction of the ply. 

Assembly and Gluing 

The glues were used as far as possible in accordance with the manufacturer’s directions. 

Gluing Process 1; Glue 8.--The glue was thinned to brushing consistency with an equal volume of ethyl acetate. The glue was spread with a brush 

to both the metal and the plywood in the area of the lap. The first coat was 

allowed to air-dry at least 1 hour, then a second coat was applied. After 

the second coat, an open assembly period of 2 to 48 hours was allowed. 

Groups of 12 specimens were prepared by assembling two panels of six 

specimens each in a jig, as shown in figure 3. This assembly was set on the 

Report No. 1570 -4 
platens of an electric hot press and heated without pressure for 15 minutes 

at 325° F., after which a pressure of 300 pounds per square inch was applied 

and the heating continued for an additional 15 minutes. The assembly was 

removed from the press while hot, but a short cooling period was allowed 

before the two panels were removed from the jig (fig. 4). 

Gluing Process 2; Glues B and G.-The8- by 8-inch metal pieces were 

masked with tape (fig. 5) so as to leave exposed slightly more than 1/2 inch 

of lap-joint area at the end of the 1- by 2-inch pieces into which they were 

subsequently cut. 

Glue B was thinned to a thin spraying consistency with approximately 

two parts of a special solvent to one of glue. The amount of thinner was 

varied somewhat, according to the viscosity of different lots of glue. Four 

coats were applied by spraying to the unmasked areas of each piece, with 

about one-half hour allowed for air drying between coats. A spray pressure 

of about 45 pounds per square inch was used, the gun was held 8 to 10 inches 

from the panel, and the nozzle adjusted to give a fan-shaped, atomized spray. 

After the fourth coat was applied, the metal pieces were exposed to 

room conditions overnight, then baked for 20 to 45 minutes in an electric 

oven at 310° to 325° F. 2 The cured film was about 0.002 inch thick and 

greenish-brown in color. It was lightly sanded with No. 1/2 emery cloth. 

The 8- by 8-inch squares were next cut into 1- by 2-inch pieces, 

which were attached to the birch plywood with tape so that the primed surface 

protruded as shown at the left in figure 6. Glue G was then brush spread to 

both the cured adhesive primer and a piece of birch plywood in the area of 

the lap (fig. 6). After an open assembly period of 2 minutes, the metal and 

plywood were laid together and placed in a book press (fig. 7) between thin 

yellow-poplar veneer, sheets of rubber, and heavy birch plywood cauls. After 

the period of 10 to 15 minutes that was required to arrange six layers of 

18 specimens each in the press, pressure of 100 to 150 pounds per square inch 

was applied. The pressure was maintained overnight (approximately 18 hours) 

at room temperature of 80° to 90° F. The assembly was taken from the press, 

tapes removed, separated into 18 individual panels, and each panel sawn into 

six specimens. 

Gluing Process 3; .Glue C.--Onevolume of the glue was thinned to 

spraying consistency with 0.8 volume of a special solvent. Eight-inch 

squares were employed as in Gluing Process 2. The adjustment of the spray 

gun was the same as used in spraying Glue B, but the air pressure was 

reduced to about 10 pounds per square inch. Six double passes of the spray 

gun were made to apply enough glue to the metal and plywood to result in a 

dry film thickness of approximately 0.002 inch. A flash-off period of about 

10 minutes was allowed after each double pass of the spray. The final. 

2 The manufacturer's recommendation was 15 minutes at 300° F. but under these 

conditions the glue film did not adhere properly to the metal, and longer 

baking at high temperature was found to give better adhesion, 

Report No. 1570 -5-
coating was allowed to air dry overnight, then baked in an oven at 180° F. 

for 45 minutes. As in Process 2, the 8-inch squares were cut into the 1­

by 2-inch metal pieces. The metal and plywood were assembled together in 

the pressing jig (fig. 3), and the assembly was cured for 20 minutes at 

325° F. at a pressure of 300 pounds per square inch. 

Gluing Process 4; Glues D and E.-Thelap area of the 1- by 2-inch 

metal pieces was sprayed with Glue D at 40 pounds of air pressure. 10 double 

passes being made with the spray gun to build up a dry film thickness of 

about 0.003 inch. An air-drying period of 24 hours was then allowed for 

evaporation of solvent from the adhesive. 

Two coats of glue E were applied by brush to the plywood, allowing 

10 minutes for the first coat and 20 minutes for the second coat to air dry. 

The metal pieces were then assembled to the plywood, and the assembly was 

cured for 30 minutes at 325° F. and 100 pounds per square inch pressure. 

Gluing Process 5; Glue F.--Two coats of liquid glue F were brushed 

on the lap area of the 1- by 2-inch metal pieces, allowing approximately 30 

minutes between coats. Immediately after the second coat, the metal pieces 

were dipped into a mound of the powdered component of glue F so that a 

uniform layer of powder adhered to the liquid resin on the surface of the 

metal. Following this step, an open assembly period of 1 to 24 hours was 

allowed, then the 12 metal pieces and two plywood pieces were assembled in 

the jig (fig. 3) and placed in an electric hot press. The assembly was 

cured at 320° F. for 15 minutes at a pressure of 150 pounds per square inch 

and taken from the press without cooling. The panels were removed from the 

jig (fig. 4) after a short cooling period. 

Gluing Process 6; Glues C and G.--The 8-inch metal squares were 

masked (fig. 5), coated, and cut into small pieces, as described under 

Gluing Process 2. Glue C was thinned, sprayed, and precured on the metal 

and the walnut shims, by the same method as used in Process 3. The small 

metal pieces and walnut shims, 1/2-inch wide, were assembled on a caul of 

yellow-poplar veneer so that the adhesive coatings on the metal and shims 

were in contact (fig. 8). The entire assembly was pressed in an electric 

hot press for 25 minutes at 3250 to 3300 F. at 175 pounds per square inch. 

After gluing the metal pieces to the walnut shims, the surface of the walnut 

was lightly sanded to remove any contamination. Glue G was then applied to 

the walnut shim and birch plywood, which were glued together by the procedure 

used in bonding the metal to the plywood for Process 2 (figs. 6 and 7). 

It was evident during the preparation of the specimens with the 

walnut shims that some shims were unaccountably weak. All the shim specimens, therefore, were pretested with 500 pounds tension load, and those that 

failed were replaced to complete the sets. 

Report No. 1570 -6-
Number of Panels and Specimens 

For each glue and metal, 160 panels of 6 specimens in each were prepared, except for Process 4, glues D & E, for which only 34 panels were 

prepared with each metal. In all, a total of 11,928 specimens were prepared. 

Painting the Specimens 

The panels of each set were randomly distributed into two groups, one 

to be painted, the other to be left unpainted. To the specimens requiring 

painting were applied one brush coat of zinc chromate primer conforming to 

Federal Specification AN-TT-P-565, thinned with three parts of thinner to 

four parts of the primer, and two brush coats of aluminized glycerol 

phthalate spar varnish conforming to Federal Specification AN-TT-A-461. The 

paint was applied to the metal only and, if any paint overlapped on to the 

wood, the paint film was cut precisely to the glue joints with a knife. 

Cutting the Specimens 

After painting and numbering of the specimens, the plywood portion 

of the panel was cut to specimens of the desired dimensions (fig. 1). 

Distribution of Test Specimens 

The painted and unpainted specimens for each gluing process were 

sorted into 11 lots of 40 specimens each to be subjected to 11 exposure 

conditions, Distribution was such that only one specimen from each pair of 

panels (12 specimens) made in the same pressing operation would be Included 

in each exposure condition. Each Lot of 40 specimens was divided into 8 

groups of 5 specimens for testing after certain exposure intervals, only one 

or two specimens from each pair of panels being tested at any test period. 

Specimens glued by process 4 were included in only four exposure 

conditions, and therefore the distribution varied slightly, but the general 

distribution method was the same as for the other glues. 

Exposure Conditions 

All specimens were first conditioned for at least 1 week at 80° F. and 

30 percent relative humidity. For each gluing process and combination of 

metal and wood used, 55 specimens were tested dry as controls without 

exposure. These control specimens were selected about equally from both the 

painted and unpainted specimens reserved for the first test. The other half 

of these reserved specimens (55) along with the extra specimens resulting 

from the distribution method were mounted on a board and exposed continuously 

to the weather, 

Report No. 1570 -7-
The remaining groups of specimens prepared by the different gluing 

processes (excepting Process 4) were exposed to the following exposure 

conditions: 

1. Continuous exposure to air of 80° F. and 30 percent relative 

humidity. 

2. Continuous soaking in tap water at room temperature. 

3. Continuous exposure to air of 80° F. and 97 percent relative 

humidity. 

4. Alternate exposure for 2 weeks at 80° F. and 97 percent relative 

humidity, and 2 weeks at 80° F. and 30 percent relative humidity. 

5. Continuous exposure to air at 158° F. and 20 percent relative 

humidity. The specimens were tested after conditioning 1 week at 80° F. 

and 30 percent relative humidity, 

6. Alternate exposure for 1 day at 158° F., 20 percent relative 

humidity, and 1 day at -67° F. in a cabinet cooled with solid carbon 

dioxide. The specimens were tested after conditioning 1 week at 80° F. and 

30 percent relative humidity. 

7. Continuous soaking at exterior temperature in 100-octane gasoline. 

8. Continuous soaking at exterior temperature in 73-octane gasoline. 

9. Continuous soaking at exterior temperature in ethylene glycol. 

10. Continuous soaking at exterior temperature in isopropyl alcohol 

de-icer fluid. 

11. Continuous soaking at exterior temperature in aircraft lubricating 

oil. 

In addition to the control and exterior exposure tests, specimens 

prepared by Gluing Process 4 were exposed to conditions 1, 2, 3, and 4. 

It was planned to remove for testing five specimens of each kind from 

exposure conditions 1, 3, 4, 5, and 6 after 4, 8, 16, 24, 32, 52, and 104 

weeks: from conditions 7, 8, 9, 10, and 11 after 2, 4, 6, 8, 12, 24, and 52 

weeks; and from cordition 2 after 3, 4, 6, 7, 8, 16, and 24 weeks. Specimens 

exposed to exterior conditions are to be removed after 12, 24, 52, and 104 

weeks. 

Testing 

All specimens were tested by applying a tension force to the specimen 

in a standard plywood testing machine at a rate of 600 to 800 pounds per 

minute. 

Report No. 1570 -8-
Results 

The average shear-strength values and estimated percentages of wood 

and glue failure obtained on the unexposed control specimens and on the 

specimens exposed up to 52 weeks are given in tables 2 through 13. The area 

of failure was distinguished as to location (a) in the wood, (b) in the 

primary glue, (c) in the interface between the glue and metal, or (d) in the 

secondary glue in those processes in which a secondary glue was employed. 

There was some difference in the level of the joint strengths shown 

by the control specimens for the several processes. Gluing Processes 1, 3, 

and 4 gave the highest average values (936 to 1,000 pounds per one-half 

square inch), processes 2 and 5 the next (716 to 855 pounds per one-half 

square inch), and process 6 the least (544 to 652 pounds per one-half square 

inch). There seemed to be no consistent superiority in strength of the 

specimens made with one metal over those made with the other metal. 

Throughout this study there was little evidence that paint coatings 

applied to the metal portion of the specimen importantly improved the durability of the joint. In some instances, the effect of the exposure was 

delayed a few weeks, but even a short delay was not always evident. In 

analyzing the test data, the results on painted and unpainted specimens were 

usually so nearly indistinguishable that they were considered together. 

Continuous exposure to 80° F. and 30 percent relative humidity (table 

2).--Of the specimens glued by Processes 1, 2, and 5, there were no significant changes in joint quality during a year of storage at 80° F. and 30 percent relative humidity. The same was true for the aluminum specimens glued 

by Process 3, but with the steel specimens made by this process there was a 

drop in joint strength and wood failure even under these very mild conditions. Of the specimens glued by Process 4, there was a definite indication 

of decreasing joint quality during the year for both steel and aluminum 

specimens. The specimens glued by Process 6 showed a slight falling trend 

in joint quality, but it is not definite enough to be conclusive. 

Continuous soaking in water (table a).--The aluminum specimens glued 

by Process 1 averaged only 59 to 65 percent of the strength of the controls 

when soaked 3 weeks in water, but there was only a little further loss in 

strength after soaking for 24 weeks. There was appreciably more failure of 

the glue to the aluminum in the wet specimens than in the dry controls. The 

steel specimens glued by Process 1 averaged only 47 to 52 percent of their 

dry strength when soaked 3 weeks, but there was little change thereafter up 

to 24 weeks in water. 

The aluminum specimens glued by Process 2 averaged 60 to 72 percent 

of their dry strength when soaked 3 weeks, but there was no apparent further 

change. There was considerably more failure of the glue at the interface 

with the aluminum in the wet specimens than in the dry. The steel specimens 

averaged only 30 to 47 percent of their dry strength after 3 weeks of soaking, but there was little change thereafter. Considerably more failure 

occurred at the steel interface in the wet specimens than in the dry. 

Report No. 1570 -9-
The aluminum specimens glued by Process 3 had 56 to 66 percent of 

their dry strength when soaked 3 weeks, but no consistent change in the 

joints was apparent until between 16 and 24 weeks of soaking, when the joint 

strength dropped sharply to about one-third the dry strength value; considerable increase in the percentage of failure took place at the aluminum interface. The steel specimens glued by Process 3 had only 23 to 31 percent of 

their dry strengths after soaking 3 weeks and failed almost completely in 

the bond to the steel, but there was little apparent change thereafter up to 

24 weeks. 

The aluminum specimens glued by Process 4 had 57 to 63 percent of 

their dry strength when soaked 3 weeks and there was a considerable increase 

in the portion of the failure that occurred at the aluminum interface, but 

little change occurred between 3 and 24 weeks. The steel specimens lost 

almost all their strength in 3 weeks of soaking and failed entirely in 

adhesion of the glue to the steel. 

The aluminum specimens glued by Process 5 had 78 to 85 percent of 

their dry strength and showed somewhat greater failure in adhesion to the 

aluminum after soaking 3 weeks in water, but little change thereafter up to 

24 weeks. The steel specimens had only 30 to 40 percent of their dry 

strength after 3 weeks in water, and failed almost completely in adhesion 

to the steel. 

The aluminum specimens glued by Process 6 had 46-69 percent of their 

dry strength when soaked 3 weeks in water. and there was appreciably more 

failure in adhesion of the glue to the metal in the wet specimens than in 

the dry, but there was little further change between 3 and 24 weeks in 

water. The steel specimens retained only 10 to 22 percent of their dry 

strength after 3 weeks in water and failed completely in adhesion to the 

steel. 

Continuous exposure to 97 percent relative humidity (table 4). --The 

aluminum specimens glued by Process 1 fell to about 65 percent of their dry 

strength on becoming moist in the air of high relative humidity, but showed 

only slow or no decline thereafter for several months. Between 32 and 52 

weeks, joint strength dropped sharply because of deterioration of the wood, 

as evidenced by high wood failure. The steel specimens showed similar 

behavior with less evidence of deterioration of the wood at 52 weeks. 

Both aluminum and steel specimens glued by Process 2 lost strength on 

becoming moist in high relative humidity, but lost little strength thereafter until the wood began to fail. 

Both aluminum and steel specimens glued by Process 3 showed progressive loss in shear strength which was undoubtedly influenced, particularly 

in the latter part of the year, by deterioration of the wood. 

Report No. 1570 -10-
--- 

The aluminum specimens glued by Process 4 lost strength when they 

took up moisture in the air of high relative humidity, but showed little 

change thereafter up to 52 weeks. The steel specimens showed, in general, 

a progressive loss in strength with time of exposure and increase in the 

amount of adhesion failure of the glue to the steel. 

With both aluminum and steel specimens glued by Process 5, there was 

relatively little change in joint strength in this exposure, although there 

was a general increase in adhesion failure to the steel. 

Both the aluminum and steel specimens glued by Process 6 lost 

strength when they took up moisture at the high relative humidity, but 

showed little change thereafter except for the changes that were probably 

due to deterioration of the wood. The failure in adhesion was high for the 

steel specimens after they had been exposed to the high humidity for a few 

weeks. 

Alternate exposure, to 97 and 30 percent relative humidity {table 5). 

The aluminum and steel specimens glued by Processes 1 and 2 showed little 

change during the year of exposure to this cycle. 

The aluminum specimens glued by Process 3 showed little change in 

this exposure over 1 year; the steel specimens showed a moderate decrease 

in strength of about 22 percent and a gradual increase in failure between 

the glue and the steel. 

The aluminum specimens glued by Process 4 showed a moderate to considerable decrease in the strength of the specimens without a significant 

change in the nature of the failure. The steel specimens showed a considerable drop in strength, with increase in failure between glue and steel. 

The aluminum and steel specimens glued by Process 5 showed no very 

great change under these cyclic conditions during the year of exposure. The 

aluminum specimens glued by Process 6 showed little change in 1 year in this 

cycle. The steel specimens dropped in strength and increased in adhesion 

failure to the steel. 

Continuous exposure to 158° F. and 20 percent relative humidity 

(table 6).--In this exposure, the wood appeared to be reduced in strength 

by the high temperature, and the percentage of wood failure, therefore, 

increased except in those cases where the glues were affected by the 

expo sure. 

The aluminum specimens glued by Process 1 showed a moderate drop in 

strength with an increase in percentage of wood failure. The steel specimens showed little or no significant drop in strength but a slight to 

moderate increase in failure between glue and steel. 

Both the aluminum and the steel specimens glued by Process 2 showed 

a considerable drop in strength and increase in the percentage of wood 

failure, but not an increase in the adhesion failure to the metal. 

Report No. 1570 -11-
The aluminum specimens glued by Process 3 showed a considerable drop 

in joint strength and an appreciable increase in adhesion failure in the 

year at this exposure. The steel specimens, both painted and unpainted, 

rusted badly and lost practically all their strength in 1 year, failing at 

the interface between glue and steel. 

In the aluminum specimens glued by Process 5, there was a slight 

drop in strength at the end of the year and an increase in wood failure, 

but no apparent change in the glue. With the steel specimens, there was a 

slight increase in the amount of failure in adhesion to the metal, but 

otherwise little change. 

For the aluminum specimens glued by Process 6, there was a large 

drop in strength and an increase in the failure that occurred in the 

primary glue (Glue C) and at the interface between Glue C and the metal. 

The steel specimens rusted badly and failed completely at the metal interface after a year's time, with practically no strength left. 

Alternate exposure to 158° F. and -67° F. (table 7).--The specimens 

exposed to this alternately hot and cold cycle behaved in general like the 

specimens subjected continuously to the hot, humid exposure, but with less 

change. 

Continuous soaking in 100-octane gasoline (table 8). --Both aluminum 

and steel specimens glued by Process 1 showed some increase in joint 

strength during immersion for 2 to 6 weeks, then a sharp drop. At the end 

of the year, the specimens failed almost entirely in the glue. 

Other than a slight increase in tendency to fail in adhesion to the 

metal, there was practically no change in the specimens glued by Process 2 

that were soaked in this high-octane gasoline. 

Both aluminum and steel specimens glued by Process 3 were affected 

by the gasoline, so that there was a large drop in joint strength and wood 

failure. After a few weeks, the specimens were failing largely in the 

glue. At the end of the year, the steel specimens failed to a considerable 

extent in the interface at the steel. 

Very little change was caused by the gasoline in, the specimens glued 

by Process 5. 

The specimens glued by Process 6 showed a slight drop in 2 to 4 

weeks in the gasoline, with an increase in the failure of the primary glue, 

but little change thereafter. 

Continuous soaking in 73-octane gasoline (table 9).-Gasolineof 

'73-octane rating had less effect on the specimens glued by Process 1 than 

the 100-octane gasoline. Both aluminum and steel specimens showed some 

increase in the percentage of failure that occurred in the glue after 

soaking 1 year. The gasoline treatment produced an increase in shear 

strength for 24 weeks, after which there was a sharp decline. 

Report No. 1570 -12-
The specimens glued by Process 2 showed little or no significant 

change during the year's immersion in gasoline. 

For the specimens glued by Process 3 there was a considerable drop 

in joint strength in 52 weeks, and increased glue failure. 

The specimens glued by Process 5 showed little or no significant 

change during the year's immersion in gasoline. 

The specimens glued by Process 6 lost moderately in joint strength 

by soaking in gasoline for a year. The gasoline inoreased the failure 

within the primary glue (Glue C). Toward the end of the year, much of the 

failure in the steel specimens was at the interface between the glue and 

the steel. 

Continuous soaking in ethylene glycol (table 10).--The specimens 

glued by Process 1 showed a moderate falling off in joint strength and a 

slight inorease in the tendency to fail in the glue upon soaking 1 year in 

glycol. 

The specimens glued by Process 2 also showed some falling off in 

joint strength but no significant change in the nature of the failure. 

The specimens glued by Process 3 showed a considerable decrease in 

joint strength upon soaking 1 year in glycol but no very significant change 

in the nature of the failure. 

The effect of the glycol on the specimens glued by Process 5 appeared 

to be small and perhaps without significance. 

The changes in the aluminum specimens glued by Process 6 appeared to 

be small and without significance. The steel specimens, however, showed a 

slight decrease in strength but a considerable increase in the amount of 

adhesion failure. 

Continuous soaking in isopropyl alcohol (table 11).--Glue A was 

softened by the isopropyl alcohol and the specimens bonded by Process 1 

failed at low strengths almost entirely in the glue after 6 to 8 weeks of 

soaking. 

Isopropyl alcohol apparently had no effect on Glues B and G, as 

there was no significant change in the specimens glued by Process 2. 

Glue C may be slightly affected by isopropyl alcohol, for the specimens glued by Process 3 showed in general some decrease in strength towards 

the end of the year and an appreciable increase in glue failure. 

There was apparently no effect by the isopropyl alcohol on the 

specimens glued with Glue E by Process 5. 

Report No. 1570 -13-
The data on specimens glued by Process 6 also indicate slight effect 

by the alcohol on Glue C. There was a decrease in the amount of failure 

that occurred in the wood with the soaked specimens, and an appreciable increase in adhesion failure in the steel specimens at the end of the year Of 

immersion. 

Continuous soaking in aircraft lubricating oil (table 12) From the 

results on the specimens glued by Process 1, the lubricating oil may have a 

slight effect on Glue A, but it was not revealed until the tests were made 

at the 52-week period and not consistently at that time. 

There was no apparent effect of the lubricating oil on the glues of 

Process 2. 

The results on specimens glued by Process 3 indicate that there may 

be a slow effect of the oil on Glue C, but the effect in a year of immersion 

was slight. 

The results on specimens glued by Process 5 indicate that there may 

be a slow effect of the oil on Glue F, for some of the tests made at the end 

of the year showed an appreciable drop in shear strength and an increase in 

adhesion failure. 

The results on specimens glued by Process 6 again indicate some slow 

effect of the oil on Glue C, but it was not revealed until tests were made 

at 52 weeks and not consistently at that time. 

Continuous exposure to weather (table 13) .--The unpainted aluminum 

specimens glued by Process 1 showed a moderate drop in strength and a considerable increase in adhesion failure during the year outdoors. The painted 

aluminum specimen and the steel specimen, painted or unpainted, showed little 

change that was significant. 

The aluminum specimens glued by Process 2 showed little change during 

the year but the steel specimens showed an appreciable drop in strength and 

increase in adhesion failure. 

The specimens glued by Processes 3 and 4 showed some drop in joint 

strength during the year's exposure without a consistent change in the nature 

of the failure. 

The specimens glued by Processes 5 and 6 showed no significant changes 

during the year's exposure outdoors. 

Report No. 1570 -14-
Table 1. --Designation of gluing processes and glues 

Report No. 1570 












Figure l.--Plywood-to-metal glue-joint test specimen with and without walnut shim. 

ZM 92928 F 



Figure 6.-Applying the secondary glue to the primed metal surface in the preparation of the 

wood-to-metal glued joint specimens. 

Z M 73510 F 
Figure 7.--Press used for the secondary gluing of the woodto-metal joint specimens. 

Figure 8.--Taping walnut shims and metal pieces to 

yellow-poplar cauls. 

Z M 73511 F